The weak force is one of the four fundamental forces of nature, along with gravity, electromagnetism, and the strong force. The weak force is the second weakest of these, after gravity. It affects all matter particles, but not force-carrying particles. Weak interactions are responsible for all processes in which flavor changes, and hence for the instability of heavy leptons and quarks, and particles that contain them. Weak interactions that do not change flavor have also been observed.